This invention relates to processes and subsystems for coating thin plastic substrates with thick metallic layers.
When a metallic coating material is applied to a component or substrate, heat is generated. For substances not susceptible to thermal damage, the deposition rate is not usually a factor. And, for insulative plastic parts, if the coating is applied only as a very thin layer, thermal damage will not occur because the dwell time is short.
To place a thick (e.g., 2-4 micron) metallic layer, on a thin (e.g., 1-4 mm thick) plastic substrate, however, the dwell time is long enough for the heat generated during this process to cause thermal damage to the plastic part and/or cause it to become brittle.
Sputtered coatings are applied to highly insulative plastic parts for various reasons. Dielectric coatings are applied to modify surface properties such as hardness, environmental susceptibility, and optical transmission. Metal coatings are applied for optical reflectance, EMI/RFI attenuation, and optical transmission. Other coatings and applications exist which are not specifically outlined here, but which may also be applied through sputtering. In one example, the inside of a cellular telephone housing is metalized for shielding purposes. In another example, plastic eyeglass lenses are coated with an anti-reflective and/or protective (anti-scratch) coatings.
Present methods of coating plastic parts with metal are electroplating, electroless plating, painting, arc-spray, evaporative vacuum metalization, and sputter vacuum metalization. These processes are usually batch oriented and require a large production run volume to be cost effective. Batch processes result in large work in process inventories (WIP) and logistical complications in scheduling and part management. In-line sputtering systems (as is common in optical disc manufacturing) eliminate issues such as long cycle times and high WIP, however, they require short sputtering cycles to properly interconnect with other production processes.
Rapid deposition of thick sputtered coatings, however, is limited by part overheating as explained above. Generally, as the sputtered coated thickness increases, heat is deposited into the underlying substrate. Since sputtering occurs in a vacuum, the primary modes of heat transfer (conduction and convection) are substantially reduced because there is little gas available to transport heat to the surroundings. Radiation heat transfer provides little benefit since radiation heat fluxes are well below the heat flux imposed on the part from the incoming sputtered film.
Because of this limitation, sputter coating of thick layers requires long deposition times and low deposition rates to prevent part overheating because the incoming heat flux from the sputtered layer must be reduced to a level where part overheating does not occur.
Production engineers realize that the coating must be applied to the part quickly to meet the demands of the production cycle but also realize that thermal damage can then occur due to overheating. So, they continuously walk a fine line between speed and thermal damage. Often, however, the line is crossed resulting in defective parts, and the shutdown of the production facility to troubleshoot the system. In the case of cellular telephone housings, 5,000 parts are often treated at once in a large vacuum chamber. If the parts overheat, the result is a waste of money and time. On the other hand, if the dwell time is slowed down to prevent overheating, not only are the demands of the production cycle not met but contamination of the parts can occur when, because of imperfect vacuum conditions, contaminants enter the vacuum chamber and contaminate the parts.
It is known in the art of physical vapor deposition processes relating to coating aluminum magnetic discs with various coatings to lower the temperature of the magnetic discs in the vacuum chamber. Before chromium, cobalt alloys, or carbon layers can be applied to the aluminum disc, it must be cooled from 300xc2x0 C. to 150xc2x0 C. and cooling is effected by moving the aluminum disc between two planar heat sinks. Even by using cyrogenic heat sinks, this temperature reduction takes over a minute. See U.S. Pat. Nos. 5,287,914 and 5,181,556 incorporated herein by this reference.
Temperatures as high as 150xc2x0 C. and dwell times as long as a minute, however, will cause thermal damage to insulative plastic parts. Therefore, unlike the art of cooling aluminum magnetic discs in which thermal damage is not a primary concern and instead cooling is effected only because some of the coating processes are temperature dependent, thermal damage and embrittlement are the primary focus in the art of coating relatively thin plastic components with thick metallic layers.
It is therefore an object of this invention to provide a method of applying a coating to an insulative substrate.
It is a further object of this invention to provide such a method which prevents thermal damage, embrittlement, product waste, and the troubleshooting required when plastic parts are completely coated at once with a metalization layer.
It is a further object of this invention to provide a method of rapidly and actively cooling plastic substrates between sputtering steps without affecting the quality of the overall coating.
It is a further object of this invention to provide such a method in which thermal damage is avoided and yet the overall process is still almost as fast as the prior art method when all parts were completely coated during only one coating step.
It is a further object of this invention to provide a system for applying a coating to an insulative component.
It is a further object of this invention to provide such a system which includes at least one active cooling station arranged to actively cool the insulative components and drive the temperature of the insulative components substantially down between sputtering steps to prevent thermal damage.
This invention results from the realization that the thermal damage, embrittlement, product waste, and troubleshooting required when plastic parts are completely coated at once with a metalization layer can be avoided by only partially coating the parts and, before thermal damage can occur, moving the parts to a cooling station where they are actively and quickly cooled and then repeating the coating and active cooling processes until the desired coating thickness is obtained. Thermal damage is avoided and yet the overall process is still faster than the prior art method when all the parts were completely coated during only one coating step.
This invention features a method of applying a coating to an insulative substrate comprising the application of a coating material to the insulative substrate by physical vapor deposition to a predetermined thickness at a rate and for a predetermined time which does not cause thermal damage to the insulative substrate. Then, before thermal damage can occur, the partially coated substrate is moved proximate an active cooling station device to drive the temperature of the insulative substrate substantially down. The coating and cooling steps are then repeated until the desired coating thickness is obtained to avoid thermal damage to the substrate.
The coating material is typically metal such as copper, aluminum and alloys of the same, and the same material is usually applied during all coating steps. The coating material may also be a polycrystalline substance. The typical substrate is plastic between 1-4 mm thick. Preferably, coating is applied to a total thickness N, there are X coating steps, and, at each coating step, a thickness of N/X is applied. In one example, N is between 2 to 4 microns and N/X is between xe2x85x9 and ⅝ micron.
The insulative substrate may be the housing of an electronic device such as a cellular telephone. Another insulative substrate is a plastic lens.
Physical vapor deposition may include sputtering, cathodic arc deposition, and evaporation techniques. In the preferred embodiment, cooling includes placing the partially coated substrate proximate a heat sink and subjecting the partially coated substrate to a high conductivity gas such as helium. The heat sink is typically cooled by a liquid coolant.
Each coating step preferably lasts less than one minute and each cooling step also lasts less than one minute. To balance the system, the time span in which each partial layer of coating material is applied is the same or approximately the same time as the time span for each cooling step. Preferably, substrate temperature never exceeds between 60xc2x0 C. and 90xc2x0 C. The cooling step typically drives the temperature of the substrate from between 40-60xc2x0 C. to between 5-20xc2x0 C.
This invention also features a system for applying a coating to an insulative component. There is a vacuum chamber, at least one physical vapor deposition station arranged to apply a coating material to the insulative component, and at least one cooling station arranged to actively cool the insulative component and drive the temperature of the insulative component substantially down.
A component handler is designed to move the insulative component within the vacuum chamber and programmed to automatically bring the components proximate a physical vapor deposition station until the components are partially coated to a predetermined thickness and then proximate a cooling station before thermal damage can occur to the components and until they are sufficiently cooled and to then switch between physical vapor deposition stations and cooling stations until the desired coating thickness is obtained.
Typically, there are a plurality of physical vapor deposition stations and cooling stations arranged circumferentially with cooling stations positioned between physical vapor deposition stations. In another embodiment, there are a plurality of physical vapor deposition stations and cooling stations arranged linearly with cooling stations positioned between physical vapor deposition stations. Each cooling station may include a heat sink in a subchamber and means for filling the subchamber with a high conductivity gas. In one embodiment, the components included a cavity and the heat sink is then shaped to fit within the cavity. The programming of the component handler may include logic which limits the partial coating time to less than one minute and the cooling time to less than one minute and preferably the time span of partial cooling is the same as or approximately the time span of cooling. Typically, the component handler includes trays for holding a plurality of components, each cooling station includes one heat sink for each tray, and each cooling station further includes a subchamber containing all the heat sinks and means for filling the subchamber with a high conductivity gas.